Fatigue in progressive multiple sclerosis: results of a randomized, double-blind, placebo-controlled, crossover trial of oral 4-aminopyridine - Rossini, PM; Pasqualetti, P; Pozzilli, C; Grasso, MG; Millefiorini, E; Graceffa, A; Carlesimo, GA; Zibellini, G; Caltagirone, C (Mult Scler, 2001)